Various types of fluid pressure sensing devices are currently available for use as safety devices in the monitoring and sensing of process pressures such as at oil and gas wells, petrochemical sources, and associated production and processing installations. The United States Government has specified certain performance and safety requirements for such pressure sensors, known in the trade as "pressure pilots". The required sensors or pilots must be highly accurate and extremely reliable in order to properly protect production equipment, operating personnel and the environment at on-shore oil and gas drilling and processing plants as well as such off-shore installations.
Thus, presently available pressure sensors or pressure pilots are designed to automatically activate or trip safety shutdown systems when the sensed process pressure either extends above a previously set high pressure point or extends below a low pressure set point. When the process pressure returns to the desired safe range within the high and low pressure limits, the pressure sensor should be capable of automatic reset.
It is desirable that a pressure sensor be capable of accurate set point tripping under repeated conditions (this function being known as "set point repeatability"), and once tripped, should have the capability to reset within a fairly narrow zone (commonly known as "trip-to-reset"). The capability of providing high-only, low-only, or high and low pressure set points or pressure limits is desirable.
Generally, presently available pressure sensors include a Bourdon tube for sensing pressure changes, adjustable switch point means for setting a pressure set point and responding to movements in the Bourdon tube caused by sensed pressure changes, with indicating means providing an indication that the pressure set point established by the adjustable switch point means has been reached. In such units, the switch point device must be able to be readily set while also be capable of maintaining a reliable set point, even when subject to vibrations.